When the rotation of the compressor in an automotive air conditioner is stopped by the locking of a rotation member, the connection between the driving source and the compressor should be disengaged as quickly as possible in order to prevent damage to the driving parts of the automobile. Such disengagement is desirable to ensure that the operation of other equipment remains unaffected by the compressor malfunction, especially where the compressor and other auxiliary equipment, such as an alternator or the power steering, are coupled to the engine output through a single power transmission belt.
Various rotation detecting devices have been proposed which detect compressor locks by sensing changes in the rotational speed of the compressor and then disengage the driving force to the compressor when the rotational rate falls below some predetermined reference rate. One such rotation detecting device includes a magnetic flux changing portion, which varies the magnetic flux density of a magnetic configuration in response to the rotation of a drive shaft, and a magnetic detecting device, which detects the change in flux density. The construction of these prior art devices is, however, very complicated and can also be difficult to implement. In addition, such devices have reliability problems. For example, if a magnet with a relatively large magnet flux is used as the flux changing portion, it may attract iron grains from the interior of the compressor, producing unreliable magnetic flux density values. the reliability of the detecting device may also be adversely affected by temperature changes in the interior of the compressor since some magnets are temperature sensitive and can have their magnetic properties altered if the temperature rises above, or falls below, a certain level.